Declining soil fertility was a prime cause of the US Civil War according to Peter Montgomery.  In his book, Dirt, the erosion of civilizations, Montgomery says:

“The economic significance of limiting slavery’s expansion lies in the central role of soil exhaustion in shaping plantation agriculture and the southern economy . . . In the half century leading up to the Civil War southern agriculture’s reliance on slave labour precluded the widespread adoption of soil-conserving methods, virtually guaranteeing soil exhaustion . . . ”. p136

The states which depended on slavery for income exhausted the land by poor farming practices.

In those states the land owners could only continue to profit by continually expanding their farms to untouched fertile land and by breeding slaves on their farms for sale to ever-expanding farmlands needing slave labour.

Stopping the geographical expansion of slavery – as the northern states’ candidate and then elected President, Abraham Lincoln, planned – spelt the end of that business model.